Cooperative vehicle systems (CVS) can provide intelligent transportation systems services such as probe vehicle information and hazard warnings by exchanging data among suitably equipped vehicles as they travel. The sensitivity of the performance of CVS to the market penetration of suitably equipped vehicles is explained by using Monte Carlo analyses and simulations of wireless message propagation. The CVS functions are implemented by using wireless vehicle-vehicle data communications, which can be successful only when other equipped vehicles exist within wireless range to receive the messages and relay them to further vehicles. These relays may be accomplished by direct wireless transmission to nearby vehicles, but they may also be facilitated by transport relay, by which vehicles traveling in the opposite direction carry the messages and rebroadcast them to the vehicles that they pass. The effectiveness of both direct and transport relay mechanisms as a function of wireless communication range, market penetration, and traffic density and their influence on the speed of message propagation is shown. The direct relay is most effective with a high density of equipped vehicles, but when the density of equipped vehicles is low, the relays become much more dependent on the slower transport relay. When the market penetration is low early in the development of CVS, the most promising use cases are likely to be those that do not require rapid message propagation.